This invention relates to the fabrication of composite structures, and, more particularly, to the fabrication of composite structures having integrated apertures, by a winding method.
Composite materials are widely used in structural applications, including aerospace load-bearing structures. As used herein, a composite material is a material formed of fibers, preferably carbon or graphite fibers, embedded in a matrix, preferably a cured polymeric matrix. A convenient form for both the fabrication and use of the composite materials is a hollow form such as a tube, a hollow pressure vessel, or a hollow container, which may be cylindrical or non-cylindrical in cross-sectional shape. Such composite hollow structures have low weight, high axial strength and/or modulus, and high flexural strength and/or modulus.
The hollow structural form is readily fabricated by a winding operation. In a typical winding operation, a mandrel having the desired shape of the hollow form, but of smaller transverse dimensions, is provided. Bundles of the fiber material are wound over the mandrel in patterns dictated by the structural requirements of the completed hollow form. An uncured matrix polymeric precursor material, preferably in a viscous liquid form, is applied either simultaneously with the winding or after layers of the winding are completed. The mandrel with overwound composite material is placed into an oven or an autoclave to cure the matrix material. The mandrel is thereafter removed, and the cured composite structure is ready for use or, for some polymeric matrix materials, post curing followed by use.
In some instances, apertures through the wall of the hollow structure are required. Apertures in some cases are used for access to the interior of the structure. In other cases, they are used to bond or attach other structural members to the hollow structure.
The conventional practice for providing an aperture through the wall of the hollow composite structure is to fabricate the structure in the manner described above. The aperture is thereafter formed through the wall by cutting or machining techniques.
The present inventor has recognized that, while operable and acceptable for many applications, the existing technique for providing apertures through the wall of a composite structure has drawbacks in others. The cutting or machining operation leaves cut fibers at the edge of the aperture, which can adversely affect the structural performance adjacent to the aperture. There may be microcracks in the remaining fibers or in the matrix, which thereafter may lead to premature failure of the article and a poor surface quality. The cutting or machining operation is also time consuming and expensive, often requiring special tools and resulting in rapid wear of the tools.
Consequently, there is a need for an improved approach to the fabrication of composite structural shapes having apertures therein. The present invention fulfills this need, and further provides related advantages.